Plants possess tolerance mechanisms to cope with various types of environmental stresses in nature such as dehydration, high-temperature, freezing, or salt stress. Genes that respond to such various types of environmental stresses are considered to overlap, and tolerance to each stress is considered to be attained as a result of closely-related intracellular responses (Plant Physiol., 115: 327-334, 1997). However, genes that respond to each stress are different among temperature, dehydration, and salt stresses, and it is suggested that such genes have systems for separately recognizing each stress.
In the past, the present inventors have isolated and identified transcription factors that bind to a stress responsive cis-element, specifically activate the transcription of genes located downstream thereof, and impart environmental stress tolerance to plants. Examples include the DREB genes, such as DREB1A, DREB1B, DREB1C, DREB2A, and DREB2B genes, from Arabidopsis thaliana (Japanese Patent Publication (kokai) No. 10-228457 A (1998)). They reported that introduction and overexpression of the genes in a plant enabled stress tolerance to be imparted without the retardation of a plant (Japanese Patent Publication (kokai) 10-292348 A (1998)).
DREB proteins are roughly classified into the DREB1 type and the DREB2 type, and both of them have DNA-binding domains (AP2/ERF domains) that recognize and bind to the DRE sequence. DREB1 orthologs of DREB genes in various plants such as rice and maize have been studied (The Plant Cell Physiology, 45: 1042-1052, 2004); however, DREB2 types have hardly been studied, and examples of known DREB2 orthologs are limited to Madagascar periwinkle ORCAI, rice OsDREB2A, and the like.
DREB1 genes have a conserved C/SEV/LR sequence in the AP2/ERF domain, and DREB2 genes have the AEIR sequence. Such difference is considered to differentiate their DNA-binding properties (The Plant Cell, 10: 1-17, 1998).
Plants that overexpress DREB1A, which is of the DREB1 type, exhibit improved tolerance to dehydration, salt, and low temperature stresses; however, plants that overexpress DREB2A, which is of the DREB2 type, or a rice ortholog thereof, OsDREB2A, do not express a clear phenotype or improved stress tolerance (The Plant Cell, 10:1-17, 1998). That is, activation of DREB2 types, which had been known, required some modification, such as deletion of a specific region.